The present invention relates generally to methods and apparatuses for controlling electron beams of cathode ray tubes, and more particularly to a method and apparatus for controlling an electron beam in a cathode ray tube by using a deflection yoke.
An important aspect of performance for a television monitor is its ability to correctly align the individual color components of the electron beam (e.g., for a three beam electron gunxe2x80x94red, green, and blue). Convergence (or mis-convergence) describes how far apart the three electron beams spread from one another within a given pixel. Ideally, the electron beam strikes all three dots in the group without hitting any adjacent groups. Mis-convergence is a quantitative measurement of the lack of convergence of the three electron beams. A CRT with significant mis-convergence will display an image with a shadowy appearance, which can be distracting to viewers.
Typically, a deflection yoke is used to control the convergence of the electron beams (e.g., red, green and blue for a three beam system) in a cathode ray tube (CRT) by changing the winding distribution in horizontal and vertical coils to compensate for mis-convergence. For example, U.S. Pat. No. 5,838,099 discloses one such deflection yoke.
Today, customers prefer televisions with ever increasing screen sizes, wider deflection angles and flatter screen faces. These developments increase the difficulty to adjust for mis-convergence using conventional methods. Usually, mis-convergence error remains near the middle of the CRT screen. To correct for this, some have employed a dedicated correction device for use with the deflection yoke. For example, U.S. Pat. No. 5,142,205 discloses a deflection yoke having a correction circuit for correcting horizontal and vertical mis-convergence. This technique requires additional electronic components, thereby increasing the parts and assembly costs of the CRT and as well as increasing the overall dimensions of the resulting device.
There are several parameters used to quantify mis-convergence, which parameters are known as convergence parameters. FIGS. 1A-E depict mis-convergence patterns YH, VCR, PQV, S2V and S3V, respectively. These FIGS. 1A-E show the plus patterns. The dotted and solid lines ideally would lie on top of each other. Mis-convergence exists when these lines do not line up. Thus, in FIG. 1A, the red and blue lines fail to overlap. The same is true for the other FIGS. 1B-E, which represent various mis-convergence parameters. These are key parameters for vertical coils.
Normally, mis-convergences are reduced by the deflection yoke itself on the CRT. In some cases, there remain mis-convergence errors as shown in FIGS. 2A-B. FIG. 2A depicts the mis-convergence patterns prior to any adjustment. FIG. 2B depicts the mis-convergence patterns after adjustment by the vertical coils of the deflection yoke. As can be seen in FIG. 2B, there remains some mis-convergence, particularly of the S2V type. Finally, applying the correction device removes this remaining mis-convergence, as shown in FIG. 2C.
Generally, adjusting the mis-convergence about the edge of the Y-axis and each corner is performed by modification of the vertical coil by making the barrel magnetic field a bit stronger than otherwise necessary. FIGS. 2D-E depict the magnetic field created by the horizontal coil (e.g., a pincushion magnetic field) and the vertical coil (e.g., a barrel magnetic field), respectively. But there remains mis-convergences outside the above area, especially near the middle of CRT screen, which mis-convergence parameters are called S2V and S3V. These two parameters can be adjusted by using high harmonic magnetic fields elements. On the other hand, the edge of the Y-axis and corner parameter, YH and PQV, respectively, can be adjusted by using low harmonic magnetic field elements (e.g., pin/barrel magnetic field element). Previously, adjusting S2V and S3V convergence error has been accomplished by the use of a correction device placed on the deflection yoke. But there remains distorted correction due to the magnetic field created around the correction device. To correct convergence error (S2V and S3V) perfectly causes side effects for the other mis-convergence parameters. It is necessary to modify the deflection circuit and to adjust another component (e.g., horizontal linearity, geometry and so forth). See FIGS. 3A-D.
The present invention is therefore directed to the problem of developing a method and apparatus for correcting correct mis-convergence error in the middle of the CRT screen, yet which method and apparatus expand the ability to perform fine-tuning and parameter correction of a linear pattern without the need for a separate correction device, and which is applicable to both types of vertical coil configurationsxe2x80x94saddle type and toroidal type.
The present invention solves these and other problems by providing two pairs of vertical coils on the deflection yoke and a timing circuit that energizes the two pairs at appropriate times so that one pair of vertical coils can be optimized to control convergence for a particular region of the screen, while the other pair of vertical coils can be optimized for a different region of the screen.
For example, one pair of vertical coils can be optimized to correct mis-convergence often found at the edges of the screen, which pair of coils is then energized with a deflection current when the electron beam is pointing at the edge regions for which the coils are optimized. The second pair of vertical coils can then be optimized to correct mis-convergence often found in the middle of the screen, which pair of coils is then energized with the deflection current when the electron beam is pointing at the middle of the screen for which the second pair of coils is optimized.